1. Field of the Invention
The present invention relates generally to medical methods, apparatus, and kits. More particularly, the present invention relates to methods, systems, and kits for pumping blood through extracorporeal processing units and returning the processed blood to patients.
A variety of extracorporeal blood therapies exist which require blood withdrawal, passage through processing equipment, and return of the processed blood to the patient. Examples of such extracorporeal blood therapies include hemodialysis, hemofiltration, hemodiafiltration, apheresis, and the like. Access to a patient's vasculature may be provided through implanted ports, transcutaneous catheters, direct needle access into blood vessels, and other approaches. Once blood withdrawal and blood return lines have been established, the blood is pumped through an appropriate processing unit, such as a dialysis unit, filtration unit, apheresis unit, or the like and the treated blood returned to the patient.
It is easy to appreciate that careful control and monitoring of the extracorporeal blood circulation is important to both successful blood treatment and patient safety. Important parameters and conditions to be monitored and controlled include blood flow rate, line pressures upstream and downstream of the pump, blockages in the blood draw line, blockages in the blood return line, air leakage into the recirculation blood stream, and the like. Previous extracorporeal blood circulation systems have often relied on setting the speed of a peristaltic pump to control the blood flow rate. Since peristaltic pumps operate by the positive displacement of blood, it has been assumed that the flow rate will be fixed by the pump speed.
As recognized by the inventors herein, however, that assumption is not warranted. Peristaltic pumps, also referred to tube or roller pumps, rely on moving rollers to progressively “pinch” a tube to advance a series of small blood volumes through the tube and out of the pump. So long as the inlet pressure to the pump tube is generally constant, the pump output will be a predictable function of pump speed. In the case of extracorporeal blood circulation, however, where blood is being drawn through a relatively small needle or other access tube, the inlet pressure of blood to the pump can vary significantly. Moreover, the flow characteristics of a peristaltic pump may vary over time so that the volumetric output will change even if the inlet pressure remains generally constant. While use of a peristaltic pump does have a number of advantages, e.g. there are much less likely to apply a deleterious negative pressure to the blood being circulated, calculating the flow rate based on pump speed alone is nonetheless problematic.
To help monitor whether the pump is starved of inlet blood flow (which can alter the flow rate as discussed above), some prior art systems have employed pressure monitors on the blood draw and/or return lines. A fall in pressure in the draw line indicates that a blockage or other failure has occurred in the draw line, that the access needle is too small and/or that the access vessel has undergone a partial or total collapse. In contrast, a rise in pressure in the return line indicates the occurrence of an occlusion or other problem in the return line and/or the occurrence of a blockage in the vessel, access device, or fistula. In order to help assure sterility, pressure measurement has usually been performed using drip chambers where the pressure is transmitted via an isolated air line and a transducer protector to the appropriate transducer. Such drip chambers, however, increase the cost of the catheters (blood lines) used for the draw and return lines and the air interface can cause clotting, air entrapment, and other flow problems in the blood recirculation.
For these reasons, it would be desirable to provide improved methods, systems, and kits for the extracorporeal recirculation and processing of blood. In particular, it would be desirable to provide extracorporeal blood flow systems having improved blood flow rate control as well as improved capability for monitoring proper operation of the blood circulation circuit. Such systems should permit monitoring with a reduced risk of contaminating the blood or causing clotting, air entrapment, or other degradation of the blood. Preferably, such improved systems and system components will permit relatively low cost operation, and specifically will permit implementation without the use of drip chambers as required by certain prior art systems. At least some of these objectives will be met by the invention described hereinafter.
2. Description of the Background Art
U.S. Pat. No. 5,562,617 assigned to the assignee of the present application, describes a system of implantable ports and catheters for accessing a patient's vasculature, which system could be used together with the extracorporeal blood recirculation systems of the present invention. U.S. Pat. No. 4,181,132, describing an extracorporeal processing and blood circulation unit which is attached to a patient's vasculature through an implanted port. Co-pending applications assigned to the assignee of the present invention and including related subject matter include. These patents and pending applications are incorporated herein by reference.